In recent years there has come about a demand for a reduced-size substation, and this demand, on the part of public utilities, has been met by gas-insulated substation equipment, such as set forth in U.S. Patent nos. 3,378,731 -- Whitehead; 3,348,001 -- Upton et al. 3,801,768 -- Meyer; 3,794,797 -- Spindle et al.; 3,356,798 -- McKinnon; 3,610,858 -- Gruber et al.; 3,599,041 -- Boersma et al.; and 3,562,460 -- Koener.
The foregoing equipment significantly reduces the space required by the high-voltage side of substations rated, for example, 115 K.V. through 345 K.V. The space reduction is accomplished by replacing the open bus and air-type terminal bushings with gas-insulated bus, filled, for example, with a highly-insulating gas, such as sulfur hexafluoride (SF.sub.6) gas, at a pressure say, for example, of 45 p.s.i.g., and thereby permitting the location of electrical equipment very closely together. This gas-insulated substation equipment has many advantages, among which are:
1. Significant reduction in space requirements both in land area and overall height. PA1 2. Added system reliability by eliminating the possibility of phase-to-phase faults, lightning strokes within the system, or contamination of insulators. PA1 3. Reduced maintenance because the closed system is isolated from its environment. PA1 4. Added personnel safety because all live parts are covered by grounded shields. PA1 5. The gas-insulated modular approach has the additional advantage, because it provides the utility user with lower installation costs, when compared with conventional, or other types of power-transmission systems. The gas-insulated system, as briefly described above, has additional design strategies, inasmuch as the high-voltage power-transmission and control equipment is compressed, so that both the space required, and the total length of bus is minimized. The power transformers may be located on outside corners of the station so as to be capable of ready removal, and the location of cable potheads is flexible, with the result that the system may be readily connected to overhead lines.
As examples of the types of ratings for such gas-insulated transmission systems, reference may be made to the specification ratings, as set forth below:
______________________________________ 115/138 k.v. Ratings General Ratings for MGT Systems SF.sub.6 at 45 psig Rated maximum voltage 145 Bil 650 60 HZ one minute withstand 310 Chopped wave Not applicable Symmetrical 3 Second Current Rating 47 ka Momentary Current 76 ka Switching Current Ratings Circuit breaker (maximum rated interrupting current) 50 ka Magnetizing current switch 35 amps Isolator No load switching only Ground switch No load switching only Continuous Current Ratings Circuit Breaker 2,500 Amperes Load-break switch 2,500 Amperes Magnetizing current switch 2,500 Amperes Isolator 2,500 Amperes Ground Switch Not applicable Bus 3,000 Amperes 230 k.v. Ratings General Ratings for MGT Systems SF.sub.6 at 45 p.s.i.g. Rated maximum voltage 242 BIL 900 60 HZ-one minute withstand 425 Chopped wave Not applicable Symmetrical 3 Second Current Rating 47 ka Momentary Current 76 ka Switching Current Ratings Circuit-breaker (maximum rated interrupting current) 50 ka Magnetizing current switch 35 amps. Isolator No load switching only Ground switch No load switching only Continuous Current Ratings Circuit-breaker 2,500 Amperes Load-break switch 2,500 Amperes Magnetizing current switch 2,500 Amperes Isolator 2,500 Amperes Ground switch Not applicable Bus 3,000 Amperes 345 k.v. Ratings General Ratings for MGT Systems SF.sub.6 at 45 p.s.i.g. Rated maximum voltage 362 Bil 1050 60 HZ-one minute withstand 555 Chopped wave Not applicable Symmetrical 3 Second Current Rating 47 ka Momentary Current 76 ka Switching Current Ratings Circuit-breaker (maximum rated interrupting current) 50 ka Magnetizing current switch 35 amps Isolator No load switching only Ground switch No load switching only Continuous Current Ratings Circuit-breaker 2,500 Amperes Load-break switch 2,500 Amperes Magnitizing current switch 2,500 Amperes Isolator 2,500 Amperes Ground Switch Not applicable Bus 3,000 Amperes ______________________________________
It is desirable to provide an improved heating means for the arc-extinguishing gas, such as sulfur-hexafluoride (SF.sub.6) gas; for example, which will, in effect, provide the proper circulation of the heated gas within the circuit-breaker modules.